An open/close control mechanism of a fuel lid makes a plunger protrude/withdraw movement by opening/closing of the fuel lid. In the open/close control mechanism of the fuel lid, a locking mechanism is used to prevent the fuel lid from opening by protrusion of the plunger while a vehicle is traveling. In the locking mechanism, when a door of the vehicle is locked, the plunger is restricted at a withdraw position immovable in synchronization with the lock. To be specific, the locking mechanism includes a fitting part which can be fit into a part to be fitted of the plunger, and the part to be fit is positioned at a place where the part to be fit cannot be fit into the fitting part when the plunger is at a protrusion position. When the plunger moves from the protrusion position to the withdraw position, the part to be fitted moves to a position where the part to be fitted can be fit into the fitting part, and then, the fitting part fits into the part to be fitted. As a result, the protrusion motion of the plunger is restricted and the fuel lid is prevented from opening while a vehicle is traveling (for example, refer to Patent Literature 1).